Such an optical pointing device is already known in the art. International Patent Application No WO 03/049018, filed in the name of the same Assignee, which is incorporated herein by reference, discloses a method as well as a device for motion detection in an optical sensing device, such as an optical mouse.
Nevertheless optical pointing devices using a Light Emitting Diode (LED) as illumination source does not present the expected tracking performances for all kinds of surfaces, in particular on wood or glass. As an alternative to a LED, some recent pointing devices utilize a laser illumination source as a solution to the surface coverage reduction problem. However, the image that results from laser illumination is dominated by interference patterns commonly referred to as speckle. Among the factors that contribute to the speckle pattern are the type of the illumination source, system optics and the surface topography.
The image formed by laser illumination has a high spatial frequency content that prevents motion detection methods of the prior art, in particular the “Peak/Null Motion Detection” algorithm disclosed in WO 03/049018, from working efficiently for all kind of surfaces. Indeed with a laser illumination source, the number of motion features seen by the photodetector device is very high. This unusually high number of detected motion features has a negative impact on the way the photodetector will detect “loss of tracking” condition, which can be detected by looking at the number of motion features of the current flash that appear to come from nowhere from the previous flash, defined as “ghost motion features”. The failure to properly detect a “loss of tracking” condition results in poor performance during some motion conditions, such as high acceleration. When the photodetector experiences high rates of acceleration, it has to dynamically adjust the flash rate of the illumination source so that it may still track and determine the correct motion. The incorrect detection of “loss of tracking” causes deficiencies to appear in motion detection.